Dihydroxy-acid dehydratase (DHAD), also called acetohydroxy acid dehydratase, catalyzes the conversion of 2,3-dihydroxyisovalerate to α-ketoisovalerate and of 2,3-dihydroxymethylvalerate to α-ketomethylvalerate. The DHAD enzyme requires binding of an iron-sulfur (Fe—S) cluster for activity, is classified as E.C. 4.2.1.9, and is part of naturally occurring biosynthetic pathways producing valine, isoleucine, leucine and pantothenic acid (vitamin B5). DHAD catalyzed conversion of 2,3-dihydroxyisovalerate to α-ketoisovalerate is also a common step in the multiple isobutanol biosynthetic pathways that are disclosed in commonly owned and co-pending US Patent Application Publication US 20070092957 A1. Disclosed therein is engineering of recombinant microorganisms for production of isobutanol. Isobutanol is useful as a fuel additive, whose availability may reduce the demand for petrochemical fuels.
High levels of DHAD activity are desired for increased production of products from biosynthetic pathways that include this enzyme activity, including for enhanced microbial production of branched chain amino acids, pantothenic acid, and isobutanol.
There is a need therefore to increase DHAD activity in lactic acid bacterial (LAB) cells to allow increased production of isobutanol and other products whose biosynthetic pathways include DHAD.